Crop competition as a strategy to control glyphosate-resistant Chloris virgata and application of biochar to control glyphosate mobility in Australian Soils

Abstract

The aim of the thesis is to provide solutions to the problems associated with the over-reliance of glyphosate herbicide application resulting in the emergence of resistant weeds like Chloris virgata grass and the consequent problem of glyphosate mobility in different soil environments. Chloris.virgata is an emerging problem in central Queensland and northern New South Wales. I hypothesised that mung bean and sorghum crops could suppress C.virgata at narrower row spacings and higher densities. Pot experiments were established for two seasons in a glasshouse. The data from experiments indicated maximum C.virgata biomass of 43-48g dry weight in both seasons, which was significantly reduced by 74 % when surrounded by 5-6 mung bean plants m-2. Similarly, field experiments in two seasons indicated sorghum as a better competitor than C.virgata. Significant reductions in C.virgata biomass were observed when surrounded by 10 sorghum plants m-2 in rows spaced 50 cm apart in comparison to 75 and 100 cm spacings. To study the problem of glyphosate mobility, the sorption behaviour of glyphosate in four contrasting soil types (Oxisol, Vertisol, Entisol, Inceptisol) amended with aged wood biochar was examined. Batch experiments were conducted using14C labelled glyphosate and liquid scintillation counting. Significant differences in biochar amended and unamended soils were only found in the Entisol soil system. The sorption behaviour of glyphosate in soil systems was the Oxisol>Vertisol>Entisol>Inceptisol. The Oxisol soil system was found to adsorb five times more glyphosate than the Inceptisol. Inceptisol soil systems showed the lowest amount of glyphosate sorption among all the soils. From the sorption studies, it was clear that the impact of biochar application was soil specific. Biochar can sorb glyphosate in Entisols and Inceptisols but application in Vertisols and Oxisols is ineffective. My thesis found that the strategies of biochar application and crop competition can contribute to overcoming some of the problems related to over-application of glyphosate.